Compound eye image pickup apparatus

1. A compound eye image pickup apparatus comprising: a plurality of optical systems including a first optical system having a longest focal length and an optical system group of a plurality of second optical systems having focal lengths different from the longest focal length; and an image pickup element including a plurality of image pickup regions corresponding to the plurality of optical systems, wherein a condition expression below is satisfied, 0.15 2 × S p 2 × P AVE ≤ N t ≤ P SUM var × 2.0 × 10 6 where p (mm) represents a pixel pitch of the image pickup element, S (mm 2 ) represents an area of the image pickup element, P AVE represents an average value of the number of pixels of each image pickup region, P SUM represents a sum of the number of pixels of each image pickup region, N t represents the number of the first optical systems among the plurality of optical systems, and var represents the number of the different focal lengths.

2. The compound eye image pickup apparatus according to claim 1 , wherein a condition expression below is satisfied, floor ⁡ ( P SUM P AVE × 1 var ) ≤ N w where N w represents the number of the second optical systems in the optical system group that have a shortest focal length among the plurality of optical systems, and floor(x) represents a floor function that gives a maximum integer not greater than a real number x.

3. The compound eye image pickup apparatus according to claim 2 , wherein the number N w is greater than the number N t when the number of the plurality of optical systems is not an integral multiple of the number var.

4. The compound eye image pickup apparatus according to claim 3 , wherein a largest number of optical systems in the optical system group having an identical focal length among the plurality of optical systems is the number N w .

5. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems include an optical system group of the first optical systems having the longest focal length.

6. The compound eye image pickup apparatus according to claim 1 , wherein a smallest number of optical systems in the optical system group having an identical focal length among the plurality of optical systems is the number N t .

8. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are arranged such that image circles of the plurality of optical systems are in touch with each other.

9. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are arranged such that image circles of the plurality of optical systems are in touch with each other along at least one of sides of the image pickup element that are orthogonal to each other.

10. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are arranged such that circles representing outer circumferences of the plurality of optical systems are in touch with each other when the circles representing the outer circumferences of the plurality of optical systems are larger than image circles of the plurality of optical systems.

11. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are arranged such that circles representing outer circumferences of part of the plurality of optical systems are in touch with image circles of other part of the plurality of optical systems, when the circles representing the outer circumferences of the part of the optical systems are larger than image circles of the part of the optical systems.

12. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are arranged such that part of image circles of the plurality of optical systems overlap each other along at least one of sides of the image pickup element that are orthogonal to each other, when the image circles of the plurality of optical systems are larger than outer circumferences of the plurality of optical systems.

13. The compound eye image pickup apparatus according to claim 1 , further comprising a plurality of image pickup units each including the plurality of optical systems and the image pickup element.

14. The compound eye image pickup apparatus according to claim 1 , wherein the plurality of optical systems are detachably mounted on an image pickup apparatus body including the image pickup element.